1. Field of the Invention
This invention relates to lead-free solder paste for use in lead-free soldering of components in the electronics industry.
2. Brief Description of Related Technology
Solder paste is a homogenous, stable suspension of solder particles in a solder paste flux and has many applications in the electronics industry, particularly in the manufacture of printed circuit boards (PCB).
Reflow soldering is used in the automated manufacture of PCB, wherein electronic components are surface mounted on PCB to which a solder paste has previously been applied by a method such as screen printing, stencilling or dispensing. The PCB is then subjected to a sufficiently high temperature, to cause the solder paste flux and the solder particles to liquefy and to join the components in place on the PCB. The heat can be supplied by, for example, infrared, heated conveyor belt or convective means.
Soldering will not take place without solder paste flux, which is required to improve the coalescence of the molten solder particles and the wetting of metallic substrates. In particular, solder paste flux removes oxide layers from the solder and joint surfaces, transports reaction products away from the soldering zone, and protects the clean joint surfaces from oxidation until soldering has taken place. Solder paste flux also acts as a heat transfer medium which ensures that all parts of the joint reach a temperature above the melting point of the solder particles.
Many solder paste fluxes are based on rosin, which is a natural material obtained from the sap of pine trees and contains isomers of abietic acid. Synthetic resins may also be used. These materials may be referred to generically as resins and as used herein the term xe2x80x9cresinxe2x80x9d includes natural resins such as rosin, chemically modified rosin, and synthetic resins. To form solder paste fluxes the resins are dispersed in solvents, together with additional activators and gelling agents.
Activators are added to decompose and remove any oxide film existing in the portion where soldering is going to be carried out, and are usually organic compounds containing halides, typically amine hydrohalogenides, such as diethylamine hydrochloride or weak halide-free organic acids. The resin may be a weak acidic activator but it is also added to protect the metals from oxidation during solder paste reflow. The organic solvent dissolves and supplies the resin and activators to the portion to be soldered and is typically an alcohol, glycol, glycol ether, or ester. Most of the organic solvent is evaporated during the preheating step of soldering. A gelling agent such as ethyl cellulose or modified castor oil is used to maintain the necessary viscosity of the solder paste flux. This provides the correct physical characteristics to maintain a homogenous dispersion of solder powder particles, to provide a material that can be screen printed and to hold electronic components in place before the soldering process is carried out. It has hitherto been regarded as essential in the formulation of resin based solder paste fluxes that the resin is completely dissolved in the solvent.
Use of high boiling range solvents is desirable in the formulation of solder paste fluxes because it provides for a more robust solder paste printing process and shows improved ability to hold electronic components in place before the reflow assembly process. However, it has now been found by thermogravimetric analysis of solder paste through a typical PCB reflow process that solvent loss continues throughout reflow and that a significant amount of solvent is present in the molten resin before and even during the melting of solder alloy particles. It has been postulated that these solvents plasticise the resins so that they are more fluid during the reflow process and this has the disadvantage that it allows the flux to move away from the region where the cleaning behaviour is necessary and reduces the ability of the solder paste flux to prevent re-oxidation of the metal surfaces. As the reflow process becomes more aggressive, i.e., longer duration and/or higher temperatures, the effects of re-oxidation become more apparent.
Adding extra activators to the solder paste flux can improve the ability of the flux to withstand aggressive reflow processes such as in lead-free soldering, but significant additional amounts of metal salts will be produced and left in flux residues after reflow. If the same flux is used under less aggressive conditions there will be significant additional quantities of unused activator in the residues. Both the metal salts and the unused activator reduce the electrical integrity of the flux residues. Using additional activators and/or developing improved halide-containing activators is thus not very effective in improving the resistance of solder pastes in aggressive reflow processes. Additional activators also adversely affect the shelf life of solder pastes and reduce the resistance of the solder paste to drying during the printing process.
U.S. Pat. No. 4,419,146 is directed to the manufacture of useable fluxes from solvent/activator combinations that are not soluble in one another. Specifically, solder flux compositions are disclosed which are prepared by forming a dispersion of a flux material in a liquid in which the flux is substantially insoluble, wherein the flux material is a mixture of tartaric acid and sarcosine and the liquid is isopropyl alcohol with or without water. JP 63-123,592 is directed to a cream solder comprising a tin or lead solder alloy with a flux with a first gel flux comprising a rosin hardly soluble in organic solvent and an activator and a second gel flux comprising a rosin easily soluble in the organic solvent.
In the past most solder compositions used were of the tin-lead type which proved useful in most common applications. However, the use of lead in solder composition has come under scrutiny from environmental factions and has become undesirable in future design applications. As a result, attempts have been made to overcome the use of lead from solder compositions. Such lead-free solder compositions include tin (95% by weight) and antimony (5% by weight) solder compositions. Others include tin, copper and silver (typically 95.5% by weight tin, 4% by weight copper and 0.5% by weight silver) or tin, antimony, zinc and silver (typically 95% by weight tin, 3% by weight antimony, 1.5% by weight zinc and 0.5% by weight silver). Bismuth may also be used together with tin, antimony and silver in a range of approximately 1.0% to 4.5% by weight. Patents directed to lead-free alloys include U.S. Pat. Nos. 1,437,641, 3,607,253, 4,042,725, 4,170,472, 4,667,871, 4,670,217, 4,695,428, 4,758,407, 4,778,733, 4,806,309, 4,879,096, 4,929,423, 5,094,813, 5,102,748, 5,147,471, 5,242,658, 5,256,370, 5,316,205, 5,320,272, 5,328,660, 5,344,607, 5,352,407, 5,390,080, 5,393,489, 5,405,577, 5,411,703, 5,414,303, 5,429,689, 5,435,968, 5,439,639, 5,452,842, 5,455,004, 5,527,628, 5,538,686, 5,569,433, 5,580,520, 5,658,528, 5,698,160, 5,718,868, 5,730,932, 5,733,501, 5,755,896, 5,762,866, 5,817,194, 5,837,191, 5,843,371, 5,851,482, 5,863,493, 5,874,043, 5,918,795, and 6,231,691; European Patent Documents EP-A-0 251 611, EP-A-0 336 575, EP-A-0 629 463, EP-A-0 629 464, EP-A-629 465, EP-A-629 466, and EP-A-629 467; Great Britain Patent Document GB-A-2,158,459; Japanese Patent Documents JP-A-5050286, and JP-A-8230598; and International Patent Publication WO-A-94/2563 the disclosures of each of which are hereby expressly incorporated herein by reference.
The invention provides a lead-free solder paste which does not have the disadvantages noted above and is more suitable for use under aggressive conditions as required when using lead-free solders.
According to one aspect, the present invention provides a lead-free solder paste comprising a solder paste flux and solder alloy particles that are substantially lead-free wherein the solder paste flux comprises resin dissolved in a solvent and also includes undissolved resin particles less than 25 xcexcm in size homogenously dispersed therein to provide improved solder alloy powder coalescence and substrate surface wetting while maintaining appropriate solder paste rheology for use in printed circuit board assembly processes.
According to a further aspect the present invention provides a method for the manufacture of a lead-free solder paste wherein the solder paste flux has two or more resins comprising the steps of dissolving the more soluble resin in part of the solvent and cooling the resultant solution to about 40xc2x0 C. to 70xc2x0 C., adding a dispersion of the less soluble resin made in the remainder of the solvent to the cooled solution and adding a gelling agent and allowing the mixture to cool and mixing with lead-free solder alloy particles.
The gelling agent is added under controlled temperature and shear conditions and the mixture is then allowed to cool. Mixing is achieved with a high-speed emulsifier that provides the necessary shear and the final temperature during mixing is dependent on the solvent and gelling agent combination but is typically in the range 50xc2x0 C. to 70xc2x0 C.
Some of the resin initially dissolved also crystallises out of solution on cooling. The finished flux therefore contains (a) resin dissolved (b) resin originally dissolved but now dispersed as solid (c) resin added as a dispersion.
According to a still further aspect, the present invention provides a method for reducing the plasticisation of resin by solvents during reflow in a lead-free solder paste wherein the solder paste flux comprises resin and solvent which in addition to resin dissolved in the solvent also includes undissolved resin homogenously dispersed in the solvent.
It has been found according to the present invention that if the solder paste flux resin is chosen so that some of the resin is undissolved in the solvent system there is a significant improvement in the wetting and surface appearance of the solder joint in an aggressive reflow process. This results in the solder paste flux solvent being substantially insoluble in the resin melt during the reflow process so that the plasticising effect is significantly reduced. The lead-free solder paste according to the invention shows resistance to aggressive reflow processes without the need for high levels of activators that may compromise the electrical reliability of the residues and without compromising printing performance and storage stability.